The present invention relates to an improved nebulizer apparatus. Nebulizers, or atomizers, are devices, which generate a fine spray or aerosol, usually of liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or a suspended particulate or colloidal pharmaceutical agent for administration to a subject by inhalation. Such inhalation treatment is highly effective for conditions affecting the subject""s respiratory organs. Further, since the lungs are close to the heart and the blood circulatory system of the body, drug administration by inhalation provides an effective and rapid delivery system to all organs of the body.
In many cases, the nebulizer is placed directly in the mouth or nose of the subject so that the spray can be entrained in the respiratory gases inhaled during normal, spontaneous breathing of the subject. In other cases, the subject breathes with the aid of a respiratory ventilator. A typical ventilator has a breathing circuit comprising an inhalation limb and an exhalation limb connected to two arms of a Y-connector. The third arm of the Y-connector is connected, via a patient limb, to a mouthpiece, mask or endotracheal tube for the subject. The ventilator provides a complete or partial supply of respiratory gases to the subject through the inhalation limb during inhalation. The contraction of the subject""s lungs discharges gas through the exhalation limb during exhalation. When a nebulizer is employed in conjunction with a ventilator, it is typically placed in the patient limb.
Nebulizers currently in use for ventilator applications generate the spray either pneumatically or by means of ultrasonic vibrations. Pneumatic nebulizers are typically used with a liquid, such as an aqueous drug solution. High pressure driving gas is conducted through a nozzle to draw the drug from a drug supply for the nebulizer. The drug is discharged against a baffle or other means in a gas space of the nebulizer, breaking the liquid into a fine spray. The gas space is in fluid communication with the inhaled gas pathway of the breathing circuit so that the gas flow expelled from the nozzle along with the nebulized drug is conducted to the breathing circuit and ultimately to the subject.
Disadvantages in the use of pneumatic nebulizers include the following. If the nebulizer adds a significant quantity of gas, for example, up to five liters/minute, into the breathing circuit, the breathing gas composition is affected. The driving gas is most often either oxygen or air and, particularly when a ventilator is used in the treatment of a child, the gas flow from the nebulizer may form a major portion of the inhalation gas flow. Because of the gas flow from the nebulizer, control over the inhalation gas composition is lost. Also, due to passage of the driving gas through the nozzle, impingement of the drug on the baffle, etc., pneumatic nebulizers are noisy. This may contribute to the discomfort of the subject. And, as controlling the commencing and stopping of a drug agent spray is difficult and is not very accurate, pneumatic nebulizers are commonly active during both inhalation and exhalation. This obviously decreases the efficiency of drug delivery as measured by ratio of the amount of drug supplied to the nebulizer and the amount of drug actually delivered into the subject""s air ways.
In an ultrasonic nebulizer, the fine spray is produced by ultrasonic vibration of the liquid, as by a piezoelectric crystal. The liquid is dropped on, or otherwise applied to, the crystal. The on-off operation of such nebulizers is easier to control than for a pneumatic nebulizer. However, prior art ultrasonic devices require a large electrical power consumption to power the crystal and may not be able to nebulize colloidal or particulate suspensions. Partly due to the high power consumption of ultrasonic nebulizers, the equipment tends to be bulky. This can cause considerable difficulties, given the crowded environment that may surround a subject, such as a critical care patient.
U.S. Pat. No. 5,443,059, shows an attempt to solve the problem of bulkiness in an ultrasonic nebulizer. In the ""059 patent, a liquid source and metering component are provided in separate control unit that can be located at a distance from the subject. The control unit meters liquid through a feed line to a piezoelectric ceramic plate positioned in the patient limb of the breathing circuit. The piezoelectric ceramic plate nebulizes the liquid. In the event more liquid is delivered than can be nebulized, the device is equipped with a collection vessel for the excess liquid. In the structure disclosed in this patent, the metering line for the liquid to be nebulized is located above the vibrating crystal so that the liquid drops onto the crystal. However, this renders the ultrasonic nebulizer of this patent position sensitive. Additionally, during inhalation, the flow speed of the respiratory gases can exceed 10 m/s. Such a flow speed can draw the droplets of liquid away into the respiratory gases without the droplets being applied to the vibrating crystal for nebulization. This may render the inhalation therapy less effective, or may alter dosage rates, both of which can adversely affect the subject.
U.S. Pat. No. 3,812,854 describes a nebulizer for inhalation therapy in which the spray is generated on the front surface of a vibrating, porous body. The pores of the body form a network of passages that enable the liquid to flow through the body. The liquid to be nebulized is supplied under pressure from a liquid supply through a liquid conduit to the pores, and forced through the pores to the front surface of the porous body where it is discharged as a spray.
However, the complicated flow paths in the porous body increase the flow resistance so that high liquid pressure is required to transport the liquid through the body. To resist the forces resulting from the high liquid pressure, a thick porous body is required. But, such thickness increases the weight of the nebulizer as well as the amount of electric power required to vibrate same. Also, when used with suspensions containing suspended particulate or colloidal particles, the particles may be entrapped in the complicated flow paths through the porous body.
U.S. Pat. No. 5,478,378 describes a nebulizer in which the aerosol is formed using a mesh plate instead of a porous solid body, thereby to lessen or eliminate the foregoing shortcomings. The mesh plate has a plurality of orifices for the liquid in a reservoir. The orifices are tapered outwardly toward the outlet for the liquid. The liquid or the nozzle assembly is vibrated ultrasonically by a piezoelectric element to nebulize the liquid. The liquid reservoir is preferably permanently filled with liquid and maintained at a slight negative pressure.
An object of the present invention is to provide an improved nebulizer apparatus of the mesh plate type that can atomize both solutions and suspensions without clogging. Another object of the invention is to provide such an apparatus that has a liquid supply control means and a means for substantially equalizing pressures in the nebulizer apparatus with pressures in the breathing circuit. These means avoid pressure stressing of the mesh plate and avoid leaking of the liquid to be nebulized through the mesh plate. Yet another object of the invention is to provide such an apparatus that has a relative small size and low power consumption thereby facilitating its use. Yet another further object of the invention is to provide such an apparatus that is insensitive to the position in which it is operated. An additional object of the invention is provide such an apparatus that has fast response to activation and deactivation, enabling nebulization to be triggered in response to the subject""s breathing pattern. A further object of the invention is to provide such an apparatus that efficiently transforms all the liquid into an aerosol.
Provided in accordance with one aspect of the present invention is a nebulizer apparatus for atomizing a liquid into fine droplets. The aerosol so produced may contain medication to be delivered to breathing gases for a patient. In such an environment, the nebulizer apparatus is mounted in a breathing circuit adapter through which the breathing gases pass. The nebulizer has a housing. A mesh plate spanning a cavity in the housing has one side exposed to the gases passing through the housing. The liquid to be nebulized flows into the cavity from a liquid transport line to a rear surface of the mesh plate. A vibration means vibrates the plate or liquid to cause the liquid to pass through holes in the mesh plate to be broken up into droplets and discharged as an aerosol from the front surface of the mesh plate in the flow of breathing gases. The vibrator means for vibrating the mesh plate or liquid is preferably a piezoelectric element energized by an ultrasonic frequency power service.
The mesh plate employed in the present invention is thin as compared to the ceramic bodies used in prior art nebulizers. This permits reduced operating pressures and forces, less energy to vibrate the plate, and allows the use of suspensions since particulates in a suspension can flow through the paths defined by the holes instead of the myriad of complicated flow paths found in ceramic bodies.
The pressure at the rear surface of the mesh plate and at the front surface of the mesh plate is equalized to avoid stresses in the thin mesh plate. This equalization of pressure is accomplished by the use of a pressure channel in the housing communicating between the cavity in the housing and the breathing gas passage.
The delivery of liquid to be nebulized is preferably controlled responsive to the amount of liquid at the rear surface of the mesh plate. For this purpose, a sensor, such as a pair of electrodes spaced from the rear surface of the mesh plate, may be used for sensing the presence of liquid at the rear surface of the mesh plate by changes in impedance. When the appropriate amount of liquid has been delivered, delivery of liquid is stopped or altered to prevent undue pressure in the volume and resulting stress on the plate or liquid leakage through the mesh plate. The supply of liquid is resumed when the liquid on the mesh plate has been consumed or reduced, as detected by the sensor. In an embodiment of the invention, the mesh plate may serve as one of the electrodes. Or, an optical sensor may be used as a liquid detector.
The nebulizer apparatus may include a control valve connected in a liquid transport line for the liquid to be nebulized. This valve may be opened and closed, responsive to the sensor, to control the liquid flow through the transport line to the mesh plate.
A reservoir for the nebulizer apparatus connected to the liquid transport line stores the liquid prior to nebulization. The reservoir may be pressurized to supply liquid to the liquid transport line. Alternatively, a pump or elevated reservoir may be used to supply the liquid.
If small amounts of liquid are to be delivered, a pre-determined amount of the liquid may be stored in a two-part reservoir prior to nebulization. One part of the reservoir comprises a chamber for receiving the liquid to be nebulized. This chamber is separated from the other part of the reservoir by a flexible wall. The other part of the reservoir is pressurized with liquid or gas to supply the liquid from the chamber to the liquid supply line.
To avoid condensation on the front surface of the mesh plate in the moist breathing circuit environment, to reduce liquid viscosity, and/or to provide comfort to the patient, a heater, such as a resistor, may be incorporated in the nebulizer apparatus or liquid transport line.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.